Vegetable oils and animal fats and their by-products can contain considerable amounts of free fatty acids. Depending on the source of the raw material and level of processing or refining, free fatty acid (FFA) content may be between 0 and 100% by weight. When these materials are used for fatty acid alkyl ester (FAAE) production by base-catalyzed transesterification of mono-, di- and tri-acylglycerides (i.e., glycerides), the FFAs in the feed material are converted to soaps leading to yield losses and undesirable processing consequences (e.g., emulsion formation).
A pretreatment process may be used to reduce the FFA content in the raw materials (i.e., deacidify them) so that the glycerides can be converted to FAAEs in a base-catalyzed transesterification process. One method to reduce the FFA level in fats and oils is to remove them by distillation. This process can concentrate FFAs in a distillate stream to greater than 80 wt % while reducing the FFA level in the remaining fats and oils to as low as 0.1 wt % (or to an acid number of ˜0.2 mg KOH/g). This process reduces the overall yield of feedstock to FAAE though and generates a stream of concentrated FFA that requires finding a new end-use, further processing or disposal. Another common method to remove small amounts of FFA is by adding a base reactant such as sodium hydroxide in order to saponify the FFA to soap which allows removal by water washing and filtration.
Another pretreatment process used to convert FFA into esters is acid catalyzed esterification. The FFA esterification reaction is affected by temperature, molar ratio of alcohol to FFA, mass transfer limitations, catalyst concentration, reaction time, and reaction stoichiometry. Since esterification reactions are reversible, the reaction does not go to completion. However, these equilibrium-limited reactions can be propelled further by increasing the concentration of the reactants or decreasing the concentration of the products. The reaction can be summarized as follows:
where R and R1 denote a hydrocarbon chain.
Because the equilibrium-limited reaction does not proceed to complete FFA conversion, the reaction is often conducted in two or more stages to achieve acceptable conversions (for example, greater than 99% conversion). Further FFA conversion can be accomplished by removing water from the reaction products either continuously or between reaction stages by distillation, flash evaporation, decanting or other such means. However, additional reaction stages require capital investment for additional unit operations as well as additional operating expenses. Esterification reactions can also be aided with excess alcohol and catalyst addition, although economic factors, small incremental improvements, and additional operational complexity usually limit their effectiveness. Esterification reactions can be performed in either batch or continuous process applications.
One such esterification process converts free fatty acids to FAAEs with alcohols using homogenous catalysis (catalyst and reactants have the same phase). Homogenous catalysis provides excellent selectivity and activity. Sulfuric acid, p-toluene sulfonic acid, and other strong acid catalysts have been used for esterification, but process equipment corrosion, product contamination, and catalyst recovery, neutralization, disposal, health and safety concerns and continuous cost issues remain—especially for conversion of renewable feedstocks with high FFA content into biofuels. Furthermore, we have observed that esterification with methanol using methanesulfonic acid (MSA) as an homogenous catalyst cannot reduce the initial FFA content significantly below 1 wt % in a single stage reaction regardless of initial FFA content unless uneconomical amounts of methanol and/or methanesulfonic acid are used in conjunction with extended residence time and/or high reaction temperature. Generally 1 wt % FFA content is undesirable for a base-catalyzed transesterification feedstock, and additional processing steps are therefore required.
Free fatty acids in raw materials can also be esterified with alcohols using heterogeneous catalysis (i.e., catalytic reactions wherein the reactants and catalyst are in different phases). Heterogeneous catalysis often provides good selectivity and, unlike most homogeneous catalysts, are designed to be used for extended periods of time, which avoids the continuous operating expense of unrecoverable homogeneous catalysts. However, heterogeneous esterification activity is generally less than with homogeneous catalysts, and multiple stages or extreme operating conditions are typically required to achieve acceptable conversions. Heterogeneous catalysis is employed on a global commercial scale in the petroleum chemicals and fuels industries, for example, in which extreme operating conditions are used.
One type of solid catalyst for esterification reactions, acidic ion exchange resin catalysts, has demonstrated promising results for acid esterification under relatively mild conditions, with conversions typically greater than 95% of the original FFA in a single stage reaction. However, there are unresolved concerns about catalyst fouling, durability, stability, activity, and replacement schedule with continuous use of commercial-grade higher FFA feedstocks. In fact, we have observed that at any initial amount of FFA, esterification with methanol using Amberlyst BD-20 ion exchange resin catalyst can briefly reduce the initial FFA content below approximately 1 wt % in a single stage by carefully selecting certain combinations of methanol ratio, weight hourly space velocity, and reaction temperature. However, the FFA conversion continually decreases over time with typical commercial-grade high FFA feedstocks. Potential causes of this steady deactivation include catalyst fouling and deactivation by proteins, phospholipids, metal ions, neutralization, chemical compounds (i.e. choline), precipitation, and stress mechanisms (physical, thermal, osmotic). Since such deactivation is not acceptable for commercial operation, new strategies must be developed to continuously maintain heterogeneous catalyst activity while simultaneously promoting the acid esterification reaction.
It is technically feasible to regenerate deactivated ion exchange catalyst with strong acids (hydrochloric, sulfuric, and possibly methane sulfonic). However, catalyst regeneration requires capital and operating expenditures for additional process units and typically cannot recover the initial level of activity. Furthermore, regeneration or catalyst replacement is a time-consuming and waste-generating activity which puts normal plant production on hold and adds costs for waste disposal.
What is needed in the art are methods that improve upon the respective challenges and disadvantages posed by homogenous and heterogeneous catalyst use for esterification of carboxylic acids. One method of esterification using a dual catalyst process produces a sufficiently low FFA product stream from a reactor with predictable and stable activity over time. A dual catalyst process can also reduce the continuous operating expense of using unrecoverable homogeneous catalysts by reducing the amount of homogeneous catalyst required to obtain the advantages of homogeneous catalyst use.